P-xylene (PX) and propylene are both important and valuable raw materials essential for chemical industry. At present, p-xylene is mainly obtained by an aromatic hydrocarbon combination apparatus. A reformate containing aromatic hydrocarbons is prepared by continuous reforming of naphtha, and a PX product is then maximally obtained via units of aromatic extraction, aromatic fractional distillation, disproportionation and transalkylation, xylene isomerization, and adsorptive separation, etc. Since the content of p-xylene among three isomers is thermodynamically controlled, and p-xylene comprises only about 23% in C8 mixed aromatic hydrocarbons, the amount of recycling process is large, equipment is bulky, and the operational cost is high during the whole PX production process. Particularly, the differences between boiling points of three isomers of xylene are very small, high-purity p-xylene cannot be obtained with typical distillation techniques, and an expensive process for adsorptive separation has to be used. Propylene is mainly derived from byproducts in petroleum refineries and also from the production of ethylene by steam cracking of naphtha, or is produced by using propane as a raw material, which is prepared by processing of natural gas. p-Xylene is mainly used in the production of polyesters, and propylene is primarily used in the preparation of polypropylene and acrylonitrile as well as 1,3-propylene glycol required in the production of polyesters. The rapid development of the global economy increasingly demands for p-xylene and propylene as essential chemical feedstocks.
The preparation of aromatic hydrocarbons from methanol is a new route in non-petroleum schemes for preparing aromatic hydrocarbons. Chinese Patent CN 101244969 discloses a fluidized bed apparatus for aromatization of C1-C2 hydrocarbons or methanol and catalyst regeneration therein. With this apparatus and catalyst, the coking state of a catalyst in an aromatization reactor may be adjusted at any time so as to achieve the object of continuous and effective conversion of C1-C2 hydrocarbons or methanol, and highly-selective generation of aromatic hydrocarbons. Chinese Patent CN 1880288 describes a process for preparing aromatic hydrocarbons via methanol conversion. On a modified ZSM-5 molecular sieve catalyst, methanol is catalytically converted to products in which aromatic hydrocarbons predominate, of which the advantages are high selectivity for total aromatic hydrocarbons and flexible in process operation. U.S. Pat. No. 4,615,995 discloses a Zn- and Mn-loaded ZSM-5 molecular sieve catalyst for preparing olefins and aromatic hydrocarbons via methanol conversion. The ratio of light olefins/aromatic hydrocarbons in a product may be changed by adjusting the contents of Zn and Mn in the catalyst.
The preparation of olefins from methanol and the preparation of propylene from methanol are new routes in non-petroleum schemes for preparing ethylene and propylene. In 1976, Mobil Oil Corporation carried out a reaction in which methanol was converted to hydrocarbons on a ZSM-5 molecular sieve catalyst. U.S. Pat. No. 4,035,430 discloses a process for converting methanol to gasoline on a ZSM-5 molecular sieve catalyst; U.S. Pat. No. 4,542,252 discloses a technique for preparing light olefins from methanol on a ZSM-5 molecular sieve catalyst; U.S. Pat. No. 3,911,041, U.S. Pat. No. 4,049,573, U.S. Pat. No. 4,100,219, JP 60-126233, JP 61-97231, and JP 62-70324 disclose reactions for preparing light olefins from methanol by using ZSM-5 molecular sieve catalysts modified with phosphorus, magnesium, silicon, or alkali metal elements; U.S. Pat. No. 5,367,100 discloses a reaction for preparing light olefins from methanol and/or dimethyl ether using a ZSM-5 molecular sieve catalyst modified with phosphorus and lanthanum by Dalian Institute of Chemical Physics, wherein the overall selectivity of ethylene and propylene is only about 65 wt %, and the overall selectivity of ethylene, propylene, and butene is greater than 85 wt %.
CN 101767038 B and CN 101780417 B each discloses a catalyst and a method for preparing p-xylene and co-producing light olefins from methanol. It is indicated that the object of directly preparing three major essential chemicals, ethylene, propylene, and p-xylene, from methanol in one reaction process is achieved on catalysts modified with metals and silane. In hydrocarbon products obtained from the reaction, the selectivity of p-xylene in aromatic hydrocarbons is greater than 80 wt % or more, and the selectivity of ethylene and propylene in C1-C5 light hydrocarbons is greater than 80 wt % or more. However, the disadvantages of this method are that a cryogenic separation technique has to be used if a highly-pure ethylene product is to be obtained, and that the investment and energy consumption are both high, which directly affects the economy of this process.
Moreover, CN 101780417 B relates to a method for preparing p-xylene and light olefins (namely ethylene and propylene) via methanol conversion, which does not involve the processes of recycling ethylene-enriched C2− components and further performing alkylation with methanol to increase the production of propylene in order to further increase the production of propylene and circumvent the difficulty of ethylene separation. CN102464550 A discloses a method for co-production of light olefins and p-xylene, comprising preparing olefins by passing C4 and C5 hydrocarbons into a first reaction zone, which is a process of preparing olefins by cracking of C4 or liquefied gas, wherein the process for preparing propylene by the alkylation of ethylene and methanol/dimethyl ether is not involved either.